Apparatus and process for treatment of water

ABSTRACT

The invention relates to the treatment of water, including for example treatment in connection with hydrocarbon production operations. Silica in water produces undesirable scaling in processing equipment, which causes excess energy usage and maintenance problems. Electrocoagulation (EC) at relatively high water temperature followed by ultra-filtration (UF filtration) may be combined with forward osmosis (FO) to treat water. Water to be treated may be produced water that has been pumped from a subterranean reservoir. The treated water may be employed to generate steam. The treatment units (e.g., EC, forward osmosis, UF filtration, etc) can be configured into one system as an on-site installation or a mobile unit for on-site or off-site water treatment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 USC 119 of U.S. ProvisionalPatent Application Nos. 61/537,661; 61/537,666; and 61/537,682, all witha filing date of Sep. 22, 2011.

FIELD OF THE INVENTION

The field of the invention relates to the treatment of water, includingfor example treatment of water in connection with hydrocarbon productionoperations.

BACKGROUND

For every barrel of crude oil produced, about three to ten barrels ofwater is produced. In the oil and energy industry, water that is drawnfrom the formation is referred to as “produced water.” The injection ofsteam for heavy oil recovery has become an important enhanced oilrecovery (EOR) method. In EOR, high pressure steam is injected at a ratesufficient to heat the formation to reduce the oil viscosity and providepressure to drive the oil toward the producing wells. For EOR, steam isnormally produced in steam generators, with full steam makeup of wateris required to feed the generator. The feed water should besubstantially free of hardness, e.g., calcium and magnesium to preventscale formation in the steam generator tubes or in the oil formation,causing plugging of downhole injection lines, causing increased pressuredrop and increasing the power demand on pumps. Silica at highconcentration can also pose a precipitation problem with scaling insteam generators and associated pipelines. Since fresh water is notalways available for EOR, the treatment of produced water in the oilrecovery process becomes necessary.

It is desirable to reduce the levels of silica and hardness to improvethe efficiency of steam generators and simultaneously reduces the carbongeneration to the atmosphere by reduction of natural gas consumption inthe production of steam. Residual amount of free oil in the producedwater also causes inefficiencies in the steam generators. Oil needs tobe separated from the water for further processing, and such separationis a major issue in production operations. High efficiency reverseosmosis (“RO”) membranes reduce silica and hardness to negligibleconcentrations. The process desalinates the produced water—which furtherimproves the quality of steam produced. However, reverse osmosisprocesses are energy intensive and generate significant pumping costs.The amount of free oil in water is a deterrent in steam generationprocesses, as it may cause significant fouling of reverse osmosismembranes. Materials that may undesirably serve to decrease reverseosmosis efficiency are free oil, dissolved organics, silica, magnesiumions and calcium ions.

There is a need for alternative and improved methods to treat producedwater to avoid undesirable scale build-up within processing equipment.

SUMMARY

In one aspect, a system and method of treating produced water isdisclosed. The produced water is treated to remove contaminantsincluding but not limited to silica, hardness ions, TDS, TOC, and COD.The produced water is subject to an electrocoagulation process to removeat least a portion of the silica and hardness ions as suspended solids.A substantial portion of the suspended solids are removed by at leastone of floatation, sedimentation, filtering, centrifugation, settling,and combinations thereof, generating a pre-treated water. Thepre-treated water is further treated in a direct contact membranedistillation (DCMD) unit, generating treated water having less than 10mg/L TOC, less than 50 ppm silica, and less than 10 ppm hardness ions.In one embodiment, the DCMD unit employs cross-flow hydrophobic hollowfiber membranes.

In one embodiment, a high-temperature filtering device is employed tohandle produced water at a temperature of at least 50° C., wherein noheat exchanger is employed to remove or add energy to the watertreatment system. In one embodiment, the high-temperature filteringdevice is a ceramic ultra-filtration unit. In another embodiment, a hightemperature polymeric membrane is used in the filtering unit.

In one embodiment instead of or in addition to membrane distillation,the water treatment process includes a forward osmosis membraneseparation process to produce high quality desalinated water.

In another aspect, the invention relates to a system for treatingproduced water containing contaminants selected from the group ofdissolved organics, free oil and grease, and TDS as silica and hardnessions. The system comprises: an electrocoagulation unit for treating theproduced water to remove silica and hardness ions from the producedwater as suspended solids to generate a pre-treated produced waterstream having a LSI from −3 to 3; a filtration unit employing at least amembrane to remove free oil and grease from the pre-treated producedwater at a temperature of at least 50° C., generating a filtered waterstream with a reduced concentration of free oil and grease; a membranedistillation unit for removing at least 90% of dissolved organic contentfrom the filtered water stream with a reduced concentration of free oiland grease; a forward osmosis unit for removing at least 90% ofdissolved organic content from the filtered water stream with a reducedconcentration of free oil and grease. The units in the system areconfigured in a permutable fashion for the units to be interconnectedand interchangeable for the water treatment system to operate in: asequential mode with the individual units running sequentially; aparallel mode with at least two of the units running in parallel; acombination of parallel and sequential mode; all units online; at leastone unit online and at least one unit being idle; and combinationsthereof.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of a system/process configuration employing anEC unit in conjunction with high-temperature filtration to treatproduced water.

FIG. 2 is block diagram of another embodiment, wherein chemicalprecipitation is used in conjunction with ceramic ultra-filtration forproduced water treatment.

FIG. 3 is block diagram of a third embodiment, wherein an EC unit isused in conjunction with high temperature polymer ultra-filtration forproduced water treatment.

FIG. 4 is a block diagram showing a variation of the system/processconfiguration of FIG. 1, further comprising a direct contactdistillation membrane unit for further process treatment of the producedwater.

FIG. 5 is a block diagram showing a variation of the system/processconfiguration of FIG. 4, wherein forward osmosis is used to furthertreat the produced water.

FIG. 6 is a block diagram showing yet another variation of thesystem/process configuration of FIG. 4, wherein the produced waterstream feed is split with some untreated water being combined with thetreated water as feed to the forward osmosis unit.

FIG. 7 is a block diagram illustrating a system/process configuration totreat a produced water stream with a relatively low level of silica,with treatment in an ion exchange unit to remove hardness.

FIG. 8 is a block diagram illustrating a variation of the system/processconfiguration in FIG. 7, wherein an untreated produced water stream ismixed with the treated stream for further treatment.

DETAILED DESCRIPTION OF THE INVENTION

The following terms will be used throughout the specification and willhave the following meanings unless otherwise indicated.

“ppm” refers to parts per million. One ppm is equivalent to 1 mg perliter.

LSI refers to the Langelier Saturation index, an equilibrium modelderived from the theoretical concept of saturation and provides anindicator of the degree of saturation of water with respect to calciumcarbonate. It can be shown that the Langelier saturation index (LSI) canbe correlated to the base 10 logarithm of the calcite saturation level.The Langelier saturation level approaches the concept of saturationusing pH as a main variable. The LSI can be interpreted as the pH changerequired to bring water to equilibrium. Water with a negative LSI meansthat there is little or no potential for scale to form, with the watertypically dissolving CaCO3. If the LSI is positive, scale will typicallyform and CaCO3 precipitation will typically occur.

“Flowback water” refers to return water from fracking operations inshale gas plays.

“Fracking” may be used interchangeably with hydraulic fracturing,referring to a technique used to release petroleum, natural gas(including shale gas, tight gas and coal seam gas), or other substancesfor extraction as a result of the action of a pressurized fluid such asthe injection of water into the formation.

“Produced water” may be used interchangeably with “production water,”referring to water separated from the production of stream and gaswells, including but not limited to tar sand wastewater, oil shalewastewater, water from steam assisted gravity drainage oil recoveryprocess, and flowback water.

“Silica” (SiO2) will be used to refer generally to silica-basedcompounds.

“Absorbing” or absorption refers to a method or apparatus in whichabsorbants, such as active carbon, are used to absorb impurities in thewater.

“FPSO” (floating production, storage and offloading) vessel refers to avessel or a platform located over or near a subsea well site, anear-shore separation facility, or an onshore separation facility.Synonymous terms include “production facility” or “gathering facility.”

“Steam-generation quality water” refers to water having less than 10mg/L TOC (“total organic carbon”), less than 50 ppm silica, and lessthan 10 ppm hardness ions.

In the process of producing oil, “produced water” is generated duringoil production as a waste stream. In many instances, this waste streamcan be seven or eight times greater than oil produced at any given oilfield. Some of this water can be re-injected to the well for pressuremaintenance, some is injected to deep well for final disposal in thecase of proper aquifer conditions, and some is reclaimed for use asoilfield steam generator feed-water. Large amount of water is typicallyneeded for steam generation. Large amount of energy is needed to createsteam from water. The produced water, which is not re-injected to theproduction well such as reclaimed water for steam generation, has to betreated. Produced water has distinctive characteristics due to organicand inorganic matters, potentially causing fouling and limiting steamgenerator reliability, and ultimately oil production. The inventionrelates to improved processes and systems for the treatment of producedwater, e.g., water for use in steam generation, including ofelectrocoagulation pretreatment, lime (chemical) precipitationpretreatment, ceramic ultra-filtration, forward osmosis (FO), membranedistillation, and combinations thereof.

Produced Water Feed:

Produced water feed to the treatment process typical contains bothinorganic and organic constituents that limit the discharge options,e.g., dispersed oil, dissolved or soluble organics, produced solids,scales (e.g., precipitated solids, gypsum (CaSO4), barite (BaSO4)),bacteria, metals, low pH, sulfates, naturally occurring radioactivematerials (NORM), and chemicals added during extraction. The producedwater contains at least 1,000 mg/L TDS in one embodiment, at least 5,000mg/L TDS in a second embodiment, and at least 10,000 mg/L TDS in afourth embodiment. In some locations, the produced water may have TDSconcentrations of at least 150,000 mg/L. In terms of hardness level (asMg, Ca, Sr, Ba), the concentration may range from 200-2000 mg/L Mg; from5000 to 40,000 mg/L Ca, from 1000-10,000 mg/L Sr, and from 1000-10,000mg/L Ba.

The oil related compounds in produced water include benzene, xylene,ethyl benzene, toluene, and other compounds of the type identified inthe sample analysis shown in Table 1 and in other crude oil and naturalgas sources. The amount of TOC as free oil and grease can besubstantially higher as shown when there is an occasional process upset.Normally, the production water will also contain metals, e.g., arsenic,barium, iron, sodium and other multivalent ions, which appear in manygeological formations, as illustrated in Table 1 for an example ofproduced water from Wellington, Colo. after oil water separation in anAPI separator:

TABLE 1 Produced Water Quality Parameters after separation mg/l mg/lInorganics Total Dissolved Solids (TDS) 1200 6000 Total Hardness asCaCO₃ 30 300 Total Alkalinity as CaCO₃ 1000 4000 Chloride (Cl) 40 1000Fluoride <1 10 Phosphate (PO₄) <0.5 30 Nitrite + Nitrate-Nitrogen (NO₂ +NO₃—N)* <0.5 40 Metals Antimony (Sb) <0.005 1.00 Arsenic (As)* <0.0051.00 Barium (Ba)* 3.00 30.00 Berylium (Be) <0.0005 1.00 Cadmium (Cd)<0.001 1.00 Chromium (Cr) <0.02 1.00 Copper (Cu) <0.01 1.00 Iron (Fe)*0.10 30.00 Lead (Pb) <0.005 5.00 Manganese (Mn)* <0.005 10.00 Mercury(Hg) <0.0002 0.10 Nickel (Ni)* <0.05 10.00 Selenium (Se) <0.005 5.00Silver (Ag) <0.01 5.00 Thallium (Tl)* <0.002 1.00 Zinc (Zn) <0.005 10.00Organics Oil and grease* 20.0 200.00 Benzene* 1.00 10.00 Toluene* 1.005.00 Ethylbenzene* 0.10 1.00 Xylenes, total* 1.00 5.00 n-Butylbenzene*0.01 0.50 sec-Butylbenzene* 0.01 0.10 tert-Butylbenzene* 0.01 0.10Isopropylbenzene* 0.01 0.10 4-Isopropyltoluene* 0.01 0.10 0.01 0.10Naphthalene* 0.01 0.10 0.01 0.10 n-Propylbenzene* 0.01 0.10 0.01 0.101,2,4-Trimethylbenzene* 0.10 1.00 1,3,5-Trimethylbenzene* 0.10 1.00 0.101.00 Bromoform* <0.001 1.00

Depending on the concentration of the produced water feed, the selectedpretreatment method (e.g., chemical precipitation, electrocoagulation,etc.), and the end-use applications, in some embodiments, additives suchas complexing agents, coagulants, oxidizing agents (e.g., ozone,polyaluminum chloride), etc., can be added to the produced water feedupfront prior to or during the pretreatment step.

In one embodiment prior to water treatment, the produced water feed maypass through a screen or strainer to capture larger particulates,including large solids/particulates that may potentially damage or foulthe blades within the EC unit.

Chemical Precipitation Pretreatment Unit:

Depending on the properties of the produced water feed, chemicalprecipitation (CP) can be used as a pretreatment step for the removal ofsilica and/or hardness, with the addition of certain reagents in amountsin excess of the silica and/or hardness ions in the produced water feed.

In one embodiment for the removal of silica, the produced water is dosedwith a crystal forming compound such as magnesium oxide to removesilica, converting soluble silica to insoluble silica. The crystalforming compound forms crystals in the produced water that adsorbsilica, resulting in silica being driven or pulled out of solution andadsorbed on the formed crystals. Various crystal forming materials canbe added, e.g., magnesium oxide or magnesium chloride, which formsmagnesium hydroxide crystals that function to absorb silica in theproduced water, resulting in the conversion of silica from soluble toinsoluble form. It should be noted that in the case of magnesium, thereis an insufficient concentration of magnesium typically found inproduced water to yield a substantial amount of magnesium hydroxidecrystals. Thus, magnesium compounds are added to the produced water. Insome cases, the dissolved silica and the produced water can besubsequently removed from solution by mixing the produced water withcompounds having surface active properties to draw silica out ofsolution. Examples of such compounds are oxides of aluminum, silica andtitanium.

In another embodiment to soften water removing hardness ions, lime, sodaash and/or caustic is used in the pretreatment step. Both the lime(calcium hydroxide) and caustic are mixed with the feed water. Limeconverts carbon dioxide to bicarbonate ions and neutralizes thebicarbonate alkalinity of the produced water and removes calciumcarbonate hardness. The caustic removes magnesium hardness present inthe feed water and raises the pH of the produced water to a basic level.In one embodiment, the pH is raised to above 10.5. In many cases, the pHis maintained in the range of 10.5 to 11.5. The lime softening step canbe carried out at normal raw water temperature (cold lime process) toreduce the hardness of the produced water down to 30-50 ppm, or attemperatures near or above the boiling point (hot or warm lime process)to reduce the hardness of the produced water down to 15-25 ppm.

In one embodiment, other reagents or compounds (“coagulants”) can beadded to the produced water instead of in addition magnesium compounds,lime, caustic. The coagulants may act to destabilize the solidsgenerated during the softening process and further facilitates orenhances the separation of solids from the liquid in subsequent portionsof the process. Examples include but are not limited to ferric chloride,aluminum oxide, aluminum chloride, aluminum sulphate, polyaluminumchloride, ferrous or ferric sulfate, calcium oxide and mixtures thereof.Dosage may vary depending on the nature and characteristics of the feedwater, but in many cases, the dosage will vary in the range of 10-50mg/l.

The pH of the produced water is maintained in the range of 9.5 to 11.2in one embodiment, and between 10.0 and 10.8 in a second embodiment foroptimum precipitation of silica. Some caustic in the form of sodiumhydroxide or sodium carbonate may be added to trim the pH to a propervalue.

The total hardness of the CP treated water (by lime process) is lessthan 10 ppm in one embodiment; less than 5 ppm in a second embodiment;and less than 1 ppm in a third embodiment. In one embodiment, the LSIvalue of the treated water is 0. The total (soluble) silica in the CPtreated water (treatment by magnesium) is less than 50 ppm in oneembodiment, and less than 25 ppm in a second embodiment.

After water passes through the CP unit, the precipitates can besubsequently removed from the oily produced water stream by means knownin the art, e.g., in a solid separation unit, prior to further treatmentdepending on the application.

Electrocoagulation (“EC”) Pretreatment Unit:

In one embodiment, electrocoagulation (EC) is employed to remove silicaand hardness from the produced water instead of chemical precipitation.EC refers to a process of applying electrical current to treat andflocculate contaminants without having to add coagulations. EC consistsof pairs of metal sheets called electrodes, arranged in pairs of two,anodes and cathodes. At least one of the cathode and anode issacrificial and made from materials such as iron, aluminum, zinc, ormagnesium, with the ions thereof migrate into the electrolyte and bondwith impurities to create precipitates. In the EC, possible reactionsthat may occur on the anode surface are metal dissolution and oxygenevolution. The half-cell reactions may be any of anodic and cathodicreactions. In an example with iron being employed for the electrode, thepossible anodic reactions are metal dissolution, oxygen evolution, andoxidation of metal ion to higher oxidation state, as shown below:Fe=Fe2++2e−4Fe²⁺⁺O2+4H+=4Fe₃++2H₂O4Fe²⁺⁺O2+4H+=4Fe₃++2H₂O

The primary cathodic reactions that may occur on the cathode surface areoxygen reduction and hydrogen evolution, which may be expressed as shownbelow:O₂+4H++4e−=2H₂O2H++2e−=H₂

Ferric ions precipitate as ferric hydroxide. These ions function tocapture constituents in the produced water such as silica within theferric hydroxide complexes, generating precipitates, as shown below.Fe³⁺+3OH−=Fe(OH)₃

As it passes through the EC cell, the coagulants introduced by thepassage of electric currents through iron or aluminum electrodes in theEC chamber help reduce the concentration of silica to a low value withthe formation of precipitates. The EC process is tunable, meaning thatvariations may be introduced to adapt to slightly changed conditions. Inone embodiment by changing the amperage in the process, it is possibleto manipulate/vary the amount of silica removed.

Depending on the composition of the produced water to be treated,additives may be used if needed during the electrocoagulation. Forexample, when non-sacrificial cathodes and anodes are used, theadditives may be used to form ions to interact with solutes andparticulate matter in coagulating the impurities out of suspension andsolution. When sacrificial cathodes and anodes are used, additives maybe used to increase the conductivity of the water stream to enhanceelectrocoagulation processes. The additives may be later removed, orinvolved in the chemical processes to form precipitates. In addition, toimprove flocculation, flocculants can also be added to theelectrocoagulation. In one embodiment, with the addition of an oxidizingagent such as Fenton's reagent to the EC step, the dissolved organiccarbon content may be further reduced. Fenton's reagent is acommercially available solution of hydrogen peroxide and an ironcatalyst that is used to oxidize contaminants or waste waters.

The EC pretreatment process is quite efficient in treating producedwater from fields which have a large amount of TDS. In one embodimentwith the use of iron as one of the electrodes, as coagulation isgoverned by the amount of ferric ions released, the dosage is dependenton the amount of current in the system based on the following equation:Fe generated (mg/s)=I*M/Fn·(1000 mg/g); wherein M=Molecular weight ofiron; F=Faraday's Constant (96,485 C/mol); I=Applied current (Amps/s);n=number of electrons transferred in the reaction.

As shown, when the conductivity of the solution is high, its resistance‘R’ is low. For a lower voltage, the same current can be generated. Thepower consumption, defined as I^2*R is therefore significantly reducedfor the same amount of coagulant generated. This makes the process veryefficient for certain applications where the hardness and silica are tobe removed from sea water type salinity. Such high TDS water is commonlyseen in many carbonate type subterranean reservoirs.

In some applications, EC reduces silica at least 75% of the silica inone embodiment and as much as over 90% in another embodiment, reducehardness by about 60% to 90%, reduce dissolved organic carbon content byabout 25%-50%. Additionally, depending on the composition of theproduced water, the pH of the feed water to the EC unit can beoptionally adjusted to a pre-select pH to optimize its operation tomaximize the removal of both the silica and the hardness level. Theremoval of hardness materials such as calcium carbonate helps reducescaling of further treatment units downstream, e.g., filtrationmembranes.

In one embodiment with the use of sacrificial electrodes, some causticin the form of sodium hydroxide or sodium carbonate can be added to theproduced water feed to adjust the pH. By changing the pH conditions ofthe produced water to a pre-select basic pH, at least 90% of thehardness is removed in one embodiment, at least 95% removal in a secondembodiment, and at least 99% removal in a third embodiment. Thispre-select pH is at least 9 in one embodiment; at least 9.5 in a secondembodiment, at least 10 in a third embodiment, and at least 10.5 in afourth embodiment for the removal of at least 90% of the silica andhardness. In yet another embodiment, the pre-select pH is maintained inthe range of 7.2 to 11.5; and between 10.0 and 10.8 in anotherembodiment for optimum precipitation of silica. The EC treated water hasa silica concentration of less than 50 ppm in one embodiment; less than25 ppm in a second embodiment. The total hardness of the EC treatedwater is less than 10 ppm in one embodiment; less than 5 ppm in a secondembodiment; and less than 1 ppm in a third embodiment. The LSI value ofthe EC treated water ranges from −3 to 3 in one embodiment; a value of 0in a second embodiment; and a value of −2 in a third embodiment.

In one embodiment with an ultra-filtration step or a membranedistillation step downstream from the EC unit, EC treated watergenerates a cake layer on the membranes that is more easily cleaned thanwith treated water via other methods, e.g., using conventionalcoagulants. It is hypothesized that the cake layer formed on themembranes downstream of the EC unit are less compressible, as evolutionof hydrogen gas during the EC process makes the flocs less dense andthus easier cleaning.

Besides the easier maintenance work downstream and desirable end resultof high quality treated water for steam generation, the use of the ECunit in one embodiment results in an incremental increase in watertemperature. The produced water enters the system is at a temperature asmuch as 50° C. in one embodiment, at least 70° C. in a secondembodiment, and in the range of 80-90° C. in a third embodiment. As thehigh temperature is maintained in the EC step, then a limited amount ofheat may be needed to boil the water to create steam. Additionally, thecurrent increases the temperature of the produced water. This additionalheat aids the thermal driving force downstream desalination forwardosmosis/membrane distillation step.

After water passes through the EC unit, the precipitates can besubsequently removed from the oily produced water stream in a solidseparation unit, prior to further treatment depending on theapplication.

Solid Separation Unit:

In the solid separation unit, a substantial portion of the precipitatesare removed using means known in the art, e.g., floatation,sedimentation, filtering, and the like, using any of an incline platesettler, settling tank, centrifuge, hydrocyclones, or enhanced gravityseparation device, or a combination thereof.

In one embodiment, the treated water is passed through a clarifier toremove precipitates, sludge, etc. In one embodiment, the clarifiercomprises a settling tank, formed in the bottom of the settling tank isa sludge scraper. Once the feed water reaches the setting tank of theclarifier, solids in the form of precipitants and suspended solids willsettle to the bottom of the settling tank to form sludge. Sludge ispumped from the bottom of the settling tank. The characteristics of theproduced sludge is dependent on the characteristics of the feed waterbeing treated, such as hardness, the metals contained in the feed water,and the alkalinity of the feed water. Typically in a process treatingproduced water, the sludge comprises predominantly iron hydroxide (60%to 70%) if iron electrodes are used, with the balance comprisinginsoluble compounds derived from hardeness causing ions.

After the solid separation step to remove a substantial portion of thesuspended solids, the treated water may further pass through any of afiltration unit, a membrane distillation unit, a forward osmosis unit,or combinations thereof for further treatment. The removal rate (ofsuspended solids) is at least 80% in one embodiment, at least 90% in asecond embodiment, and at least 95% in a third embodiment.

Adsorbing Unit:

In one embodiment wherein the removal of dissolved organic carbonremoval is insufficient in the pretreatment step, e.g., EC or CPprocess, an absorbing medium such as activated carbon can be employedfor the removal for on shore applications for at least 95% removal inone embodiment, and at least 99% removal in a second embodiment. In someapplications, advanced oxidation using ozone generators/UV—peroxide(H₂O₂) may be used instead of activated carbon, such as in some offshoreapplications. After the optional step to remove dissolved organics, thetreated water may further pass through any of a filtration unit, amembrane distillation unit, a forward osmosis unit, or combinationsthereof for further treatment.

In one embodiment, the absorbing unit employs walnut shell, whereinwater flows down a bed of walnut shells where oil is adsorbed andsuspended solids are filtered. Black walnut shells have a uniqueproperty in that they have an equal affinity for oil and water. Sincethe walnut shell filters are hydrophilic, the loosely bond residual oilcan be easily separated using low pressure backwashing. The unit can bepressurized to force water through the adsorbing bed to get the desiredperformance of at least 99% removal.

Filtration Unit:

Depending on the particular application/end-use of the treated producedwater, after the CP and/or EC pretreatment and after an optionalclarification step, there may still be some residual free oil andparticulates (residual suspended solids) in the treated water. In oneembodiment, filtration is employed to remove the free oil content(“polishing” or “polishing de-oiling”) before the water can be furthertreated if needed, e.g., in a membrane distillation unit. In otherembodiments, filtration may be bypassed if a downstream treatment step,e.g., membrane distillation unit, is tolerant to residual suspendedsolids and residual dissolved organics after the electrocoagulationtreatment.

The filtration can be in succession with the treated produced water(from any of EC, CP, solid separation unit, carbon absorber) is directedthrough a number of filters in series. The filters can be staged insuccessive filtration sizes and capacity, from filters to ultra-filtersor membranes. The filters can be of the same of different types, e.g.,ceramic filtration followed by high-temperature polymeric membranefiltrations or vice versa. In one embodiment, an ultra-filtration (“UF”)unit is employed. In some applications, polymeric ultra-filtration UFmembranes may be employed. These UF membranes may be comprised ofpolyethersulfone (PES), polyacrylonitrile (PAN) or polyvinylidenedifluoride (PVDF), which may operate up to a maximum of about 40-45° C.For produced water feed with at a temperature above 50° C., e.g., atabout 80-90° C., the water temperature will need to be lowered beforesubsequent treatment in the UF step using these membrane types.

In applications handling produced water at 80° C. or more, specialhigh-temperature polymeric membranes are employed. Examples includesulfonated polyether ether ketone (PEEK) membranes as disclosed in USPatent Publication No. 20100319535, incorporated herein by reference,NAFION® membranes (sulfonated tetrafluoroethylene basedfluoropolymer-copolymer), and membranes constructed out ofpoly(phthalazinone ether sulfone) (PPES), poly(phthalazinone etherketone) (PPEK) or poly(phthalazinone ether fulfone ketone) (PPESK).

In another embodiment for handling produced water at 80° C. or more,ceramic UF membranes are employed, wherein cross-flow filtration iscarried out with the high-velocity produced water “crossflows” acrossthe face of the ceramic membrane. Suitable ceramic membrane materialsinclude titanium, alumina, zirconia, and combinations thereof (e.g.,alumina membrane with zirconia coating, etc.). Depending on the producedwater to be processed, and the end use application, the membranes can bemicro-rated and of different sizes, e.g., ranging from 0.005 mm to about0.2 mm in one embodiment, and from 0.005 μm to 0.02 μm in anotherembodiment, from 0.05-10 μm in a third embodiment, from 0.5 to 2 μm in afourth embodiment.

The oil-free water passes through the ceramic membrane (as permeate orfiltrate) while the oily waste is concentrated in a process reservoir orretained on the feed side of the membrane as retentate. With crossflowfiltration, the tangential motion of the bulk of the fluid across themembrane causes trapped particles on the filter surface to be rubbed offThus, one advantage of cross-flow filtration is that the filter cake(which can blind the filter) is substantially washed away during thefiltration process, increasing the length of time that a filter unit maybe operational. Cross-flow filtration can be a continuous process,operating continuously at relatively high solids loads without blinding,unlike batch-wise dead-end filtration.

In one embodiment, the ceramic UF unit comprises a stainless steelhousing containing ceramic membrane elements constructed from aluminumoxide and tubular in shape. Water passes along the parallel tubes fromthe feed inlet to the outlet. The surfaces of the tubes are coated witha ceramic membrane material that has a uniform pore size to providemicrofiltration or ultra-filtration. The feed stream may be introducedunder pressure at the inlet and is withdrawn as retentate at thedownstream end. Permeate passes through the membrane into the porousceramic structure. The combined permeate from all of the tubularpassageways flows through the monolith support to permeate conduitswithin the monolith that transport the permeate through slots to anexternal collection zone.

The use of ceramic membranes for the UF unit is advantageous as ceramicmembranes, being inorganic, are not as prone to fouling as some of thepolymeric membranes and requiring less cleaning as compared to apolymeric membrane, thereby reducing the amount of downtime and backwashcycles during the operation. In cleaning operations, ceramic membranesmay withstand aggressive cleaning with sodium hydroxide, unlike mostpolymeric membranes, which cannot withstand a cleaning solution pH ofgreater than 11. The abrasive resistance of ceramic membranes makes themsuitable for high total dissolved solids (“TDS”) in water, when comparedto the polymeric UF membranes. Ceramic membranes may be used for theentire pH range (0-14), thereby facilitating high pH treatment of waterin the EC process for hardness reduction.

The use of ceramic membranes or high-temperature polymeric membranesthat can withstand water high temperatures up to 130° C. (as opposed tothe maximum operating temperature of about 45° C. with some polymeric UFmembranes) allows the handling of produced water as is. Produced waterthat is very hot need not undergo any temperature reduction beforeentering the membrane module when using ceramic membranes, reducingoverall energy consumption. In one embodiment, the use of ceramicmembranes may avoid the cost of heat exchangers employed for heatintegration and reduces associated energy losses in heat integration.With the use of ceramic or high-temperature polymeric membranes in theUF unit, generating a relatively high temperature water with sensibleheat that may be gainfully utilized downstream of the UF unit.

The filtration unit can remove at least 70% of the TOC as free oil inone embodiment; at least 90% of the TOC as free oil in a secondembodiment; and at least 95% in a third embodiment, for an final freeoil level of less than 10 ppm in one embodiment and less than 100 ppm ina second embodiment. In some embodiments, treated water from theultra-filtration unit may be fed into other units in the water treatmentprocess, e.g., a forward osmosis unit or a membrane distillation unit.In yet another embodiment, the outflow from the filtration unit is firsttreated in a gas floating unit, with the addition of an agent to helpfloat the oil/particles to the top of the tank for removal prior tooptional treatment downstream.

Forward Osmosis Unit:

Osmosis is the molecular diffusion of solvent across a semi-permeablemembrane, which rejects the solute. Osmosis is driven by a chemicalpotential gradient. This gradient is caused by differences in componentconcentration, pressure and/or temperature across the membrane. In thenon-ideal case, the use of solvent activity in lieu of the concentrationaccounts for the solvent-solute interactions. At a constant temperature,the chemical potential may be defined by: μ_(i)=μ_(i) ^(o)+RT lna_(i)+V_(i)P, where is the chemical potential of 1 mol of pure substanceat a pressure P and temperature T, a_(i) is the activity of component i(1 for pure substances), R is the gas constant and V_(i) is the molarvolume _(of) component i.

The driving force is defined as the osmotic pressure of the concentratedsolution. The membrane permeable species (solvent) diffuses from theregion of higher activity to a region of lower activity. The osmoticpressure is the pressure that must be applied to a concentrated solutionto prevent the migration of solvent from a dilute solution across asemi-permeable membrane. A common application of this phenomenon is thedesalination of seawater using “reverse osmosis (RO)” using hydraulicpressure to overcome the osmotic pressure, (also, known ashyperfiltration). It is used to reverse the flow of the solvent (water)from a concentrated solution (e.g. seawater) to obtain potable water.

Osmotic pressure can be calculated from the activity (the product of themole fraction (x) and activity coefficient (γ)) of the solvent in thetwo solutions. The relationship is as follows:

${\Delta\;\pi} = {\frac{RT}{V_{i}}{\ln\left\lbrack \frac{x^{1}\gamma^{1}}{x^{2}\gamma^{2}} \right\rbrack}}$

wherein R is the gas constant, T is the temperature, V_(i) is the molarvolume of the solvent (water), x1 and γ₁, x² and γ² refer to the watermole fraction and activity coefficients in the higher activity and loweractivity solutions respectively.

In the absence of the hydraulic pressure for reverse osmosis, thesolvent flow will continue until the chemical potential equalizes inboth the feed and the draw solution. This ‘natural’ flow of solvent iscalled forward osmosis.

In one embodiment, forward osmosis (FO) is employed for the removal ofdissolved organic content in the water. In an example illustrating thedifference among FO, PRO (“pressure retarded osmosis”) and RO for thesame solvent flows of a feed (dilute solution) and brine (concentratedsolution). For FO, AP is approximately zero and water diffuses to themore saline side of the membrane. For PRO, water diffuses to the moresaline liquid that is under positive pressure (Δπ>ΔP). For RO, waterdiffuses to the less saline side due to hydraulic pressure (ΔP>Δπ). ForFO, ΔP is zero; for RO, ΔP>Δπ(osmotic pressure); and for PRO, Δπ>ΔP. Ageneral flux relationship for FO, PRO and RO for water flux from higheractivity to lower activity (i.e. FO) is as follows: J_(w)=A(σΔπ−ΔP),wherein A is the water permeability constant of the membrane, σ thereflection coefficient, and ΔP is the applied pressure difference. Thereflection coefficient accounts for the imperfect nature (soluterejection less than 100%) of the membrane. The reflection coefficient is1 for complete solute rejection.

By choosing an appropriate salt in the draw solution in the FO unit, itis possible to pull water from a feed solution of produced water. In theFO method, produced water is introduced into the FO feed chamber,wherein it is separated into a retentate stream containing contaminantsin the feed chamber and a permeate stream (depleted in contaminants suchas dissolved organics, TDS, free oil, etc.) in the FO draw chamber whichis mixed with the draw solution to form an outlet draw solution. In oneembodiment, the draw solution comprises polyvalent osmotic ions ormonovalent osmotic ions. In another embodiment, the draw solutioncomprises an alkaline earth metal salt solution with a halide. Examplesinclude but are not limited to NaCl, Na₂SO₄, AlCl₃, MgSO₄, NH₄HCO₃,MgCl₂ and mixtures thereof.

The FO process has several potential benefits over RO including but notlimited to: less membrane fouling tendencies; less membrane support andequipment used; less energy intensive process via efficient heatintegration, by treating the draw solution at a high temperature (>50°C.) to recover desalinated water; and lessening the need for severalunit operations. FO treatment is efficient in removing particulatematters and almost all dissolved constituents for greater than 90%removal of TDS in one embodiment, and greater than 95% removal in asecond embodiment. Commercial forward osmosis units are available fromvarious vendors, such as Hydration Technology Innovations of AlbanyOreg. and Oasys of Boston, Mass. Forward osmosis units may employvarious membranes. Generally speaking, forward osmosis units are lessprone to fouling than a conventional reverse osmosis unit.

Membrane Distillation (“MD”) Unit:

In embodiments wherein the reduction in the hardness and silica levelare not necessary, the produced water can be fed directly to the MD unitwithout the pretreatment step (e.g., via EC unit, CP unit, or filtrationunit). In another embodiment, membrane distillation is employed as oneof the steps after the solid separation step. In another embodiment,membrane distillation is employed as one of the steps after thefiltration step. The MD process is a thermally driven transport ofvapor, typically through a non-wetted porous hydrophobic membrane,suitable for applications in which water is the major component presentin the feed solution.

In one embodiment, “direct contact” membrane distillation (DCMD) is usedto remove the total dissolved solids and salinity in the water. In DCMD,both the warm vaporizing feed stream and the cold condensate stream(treated produced water feed) are in direct contact with the membranedistillation apparatus. The driving force for membrane distillation isthe partial pressure differential between each side of the membranepores. Both the feed and permeate aqueous solutions may be circulatedtangentially to the membrane surfaces by means of circulating pumps.Alternatively, the solution may be stirred inside the membrane cell bymeans of a magnetic stirrer. The trans-membrane temperature differenceinduces a vapor pressure differential. Volatile molecules evaporate atthe hot liquid-vapor interface, cross the membrane pores in vapor phase,and condense in the cold liquid-vapor interface inside the membranemodule. The liquid feed water to be treated by DCMD is maintained indirect contact with one side of the membrane without penetrating the drypores unless a trans-membrane pressure higher than the membrane liquidentry pressure is applied. The hydrophobic nature of the membraneusually prevents liquid solutions from entering membrane pores due tosurface tension forces. Liquid-vapor interfaces are formed at theentrances of the membrane pores.

In one embodiment, the DCMD unit employs membrane of the type asdisclosed in U.S. Pat. No. 8,167,143 and PCT patent publication WO2012/097279, the disclosures of which are incorporated herein byreference. The membrane system employs hydrophobic hollow fibermembranes in a shell casing, with the fiber comprising any ofregenerated cellulose (RC), cellulose acetate, and cellulose triacetate(CTA). In one embodiment the membrane module is configured anddimensioned to permit cross flow of the produced water (to be treated)relative to the hollow fibers. The hollow fiber module includes acentral feed distributor tube, hollow fiber membranes positioned aroundthe central feed distributor tube, end caps with ports for the flow ofsweep air, and optionally a shell casing. The central feed distributortube includes small holes to allow the removed oil to flow out radiallyon the shell side. Sweep air may be introduced into the bore of thehollow fibers in the tube side to remove permeated water vapor. Eachmembrane unit includes about 5,000 to 200,000 hollow fiber membranes inone embodiment; from 10,000 to 100,000 fiber membranes in a secondembodiment. The membrane fibers have a length of 1 to 200 inches in oneembodiment; from 5 to 100 inches in a second embodiment. The membraneshave a wall thickness ranging from 2 to 100 μm in one embodiment; from 5to 75 μm in a second embodiment; and from 10 to 50 μm in a thirdembodiment. The membranes have a surface area of about 100 cm² to about2.0 m².

In yet another embodiment, the DCMD unit employs membranes of the typeas disclosed in US Patent Publication No. US20110031100A1, thedisclosure of which is incorporated herein by reference. The membrane isof a composite hydrophilic/hydrophobic type having a high vapor flux,comprising a hydrophilic polymer layer and a hydrophobic polymer layer.In one embodiment, the membrane has a vapor flux of at least about 50kg/m²-hr. The hydrophilic polymer layer comprises any of polysulfone,polyether sulfone, polyetherimide polyvinylidenefluoride, celluloseacetate, or combinations thereof. The hydrophobic polymer layercomprising a fluorinated surface-modifying macromolecule (SMM). In oneembodiment, the SMM is poly(urethane propylene glycol) or poly(ureadimethylsiloxane urethane).

In one embodiment, after passing through the DCMD unit, the pretreatedwater contains as little as less than 5% of the original silicaconcentration; less than 25% of the original hardness causing ions suchas calcium and magnesium; less than about 5 ppm oil; and less than 50%of the original dissolved organic carbon content.

The membrane distillation apparatus may be washed in different waysusing different washing agents known to one of ordinary skill in theart. For example, a sodium hydroxide (NaOH) aqueous solution anddeionized water may be used sequentially to wash the membrane whenneeded. In another example, dilute hydrochloric acid can be used to washthe membrane when needed.

Membrane distillation may be very useful for desalination of producedwater. Membrane modules are modular and compact. The impact of salinityon water flux is minimal, since the vapor pressure decline for even a10% brine solution is only 5% of pure water vapor pressure. The producedwater may be pumped from the reservoir at temperatures of greater thanabout 50° C., and in some applications, at greater than about 70° C. Ifrelatively cold water is run on the permeate side at about 25° C., atemperature differential of about 45° C. may be used to create a drivingforce for generating considerable water flux across the membrane. In oneembodiment, the treated water on one side of the membrane is at atemperature of at least 5° C. less than the temperature of thepre-treated water. In another embodiment, the treated water is at atemperature at least 65° C. less than the temperature of the pre-treatedwater.

The DCMD unit removes at least 50% of residual silica and hardness inthe water in one embodiment; at least 80% removal in a secondembodiment, and at least 90% removal of residual silica and hardness ina third embodiment, for a steam-generation quality water or boilerquality. The chemical oxygen demand (COD) level, which is an indicationof organic levels in the water, of the water after treatment in the DCMDunit is less than 10% of the initial level before treatment in the DCMDunit. The total COD level is less than 25 mg/L in one embodiment, andless than 20 mg/L in a second embodiment. Another indication of totalorganics removal efficiency is TOC, which is expected to be less than 10mg/L after the DCMD treatment in one embodiment, and less than 5 mg/L ina second embodiment.

Ion Exchange (IE) Unit:

In some applications wherein the primary water treatment objective isremoving the hardness and with a silica level of less than 100 ppmrendered acceptable, an ion-exchange unit is employed to capture thehardness in the produced water. In the IE unit, the hardness ions are“exchanged” and bound onto the resin, thus effectively removed from thewater for least 95% of hardness removal in one embodiment; at least 98%of hardness removal in a second embodiment; and at least 99% hardnessremoval in a third embodiment.

In one embodiment, the feed to the IE unit can be desalinated water feedstream from the MD unit (or the FO unit). In another embodiment, thefeed to the IE unit is a combination stream with a first portion beinguntreated produced water feed and a second portion being desalinatedwater feed stream from the MD unit (or the FO unit) for a combined TDSof less than 10,000 ppm. In a third embodiment, the feed to the IE unitis a combination stream with a first portion being untreated producedwater feed and a second portion being desalinated water feed stream fromthe MD unit (or the FO unit) for a combined silica level of less than150 ppm. In a fourth embodiment, the feed to the IE unit is acombination stream with a first portion being untreated produced waterfeed and a second portion being desalinated water feed stream from theMD unit (or the FO unit) for a combined silica level of less than 100ppm. In a fifth embodiment, the feed to the IE unit is a combinationstream with a first portion being untreated produced water feed and asecond portion being desalinated water feed stream from the MD unit (orthe FO unit) for a combined silica level of less than 100 ppm and acombined TDS of less than 10,000 ppm.

In one embodiment for treating oil free water with TDS of less than 5000ppm, the IE unit comprises two beds of strong acid IE resin in serieswith the first bed removing the bulk of the hardness, and the second bedacting as a polisher to remove the last traces of calcium and magnesium.In one embodiment, the IE resin is a sulfonated copolymer of styrene anddivinylbenzene, which functions by exchanging sodium ions for calciumand magnesium ions. The resin can be regenerated with sodium chloridebrine.

In another embodiment for treating produced water with TDS between5000-8000 ppm, the IE unit employs two beds of IE resin in series, witha strong acid followed by a weak acid. The strong acid as the primarysoftener to remove majority of the hardness, followed by the weak acidto ensure the final softness of the water meets spec. In one embodiment,the weak acid resin is a carboxylic acid group within an acrylicdivinylbenzene matrix with a strong selectivity for calcium andmagnesium. The resin can be regenerated by treatment with HCl to removethe calcium and magnesium, then with caustic soda to convert the resinback to the sodium form.

In one embodiment with the produced water having TDS of >8000 ppm, thesystem comprises two beds in series with a weak acid followed by a weakacid bed to reduce the hardness to a level meeting spec, e.g., to lessthan 1 ppm.

System Configurations & Applications:

The produced water for treatment in the system can be from differentsources with different compositions/properties with different treatmentrequirements. The feed from different sources can be combined for aparticular suitable treatment. The feed stream can also be split asfeedstock to different treatment units in the system, e.g., in oneembodiment, some of the water is treated in one of the treatment unitssuch as the EC unit, and some remains untreated for combination with thetreated water stream from the EC unit as a new feed stream forsubsequent treatment such as in an IE unit for hardness removal. Inanother embodiment wherein the final TDS is not an important qualityfactor, produced water containing a high level of hardness is treated ina warm lime process, with the treated stream being combined withuntreated produced water from another source such that the combined TDSis less than 15,000 ppm. The combined stream can be treated in an IEunit for hardness removal to less than 1 ppm. In another embodiment, thecombined stream for subsequent hardness treatment has a total TDS of<5,000 ppm.

The above described water treatment units (e.g., EC unit, solidseparation unit, filtration unit, DCMD unit, etc.) can be configuredinto one system as an on-site installation, or as a mobile unit for useon-site or off-site. Mobile herein means that the water treatment systemcan be moved from one location to another, e.g. distances of at least0.1 miles between the locations. In one embodiment, the units areinstalled on a converted tanker that can move (sail) from one FPSO oroff-shore production unit to another for the treatment of producedwater. In another embodiment, each unit is designed and configured foreasy access on a trailer (or box trailer, or a cargo trailer) as amodular unit that can be mobile (transportable), reused, interchangeablefor assembly according to various configurations, forming a complete“water treatment plant” or deployed as stand-alone units.

For remote onshore drilling or production environment, offshoreproduction facilities, or smaller sized production facilities such assingle service processing of parcels of less than 300,000 barrels offluids (produced water) per day, routine service or the building of atailored produced water treatment system can be expensive and may noteconomically feasible. The system employing the modular units can bescaled to the appropriate size, transported from one facility toanother, and at the destination, constructed (interconnected) per aparticular modular design so as to be functional and particularlysuitable for these facilities.

The modular systems can be assembled with the individual units beinginterconnected according to tailored configurations suitable for thetreatment of the produced water at the facility, e.g., a modular systemincluding EC unit, a modular system without EC unit, a modular systemusing CP unit (instead of EC), a system employing ceramic and polymericmembranes, multiple modular systems running in parallel or multipleunits within a system running in parallel to handle larger treatmentloads. In one embodiment, the modular unit contains at least one ofeach: EC unit, solid separation unit, high-temperature UF unit(employing ceramic or high-temperature polymeric membrane), membranedistillation unit (e.g., DCMD, VMD or vacuum membrane distillation,etc.), and forward osmosis unit.

The modular system can be configured to run in serial (sequential) modein one embodiment, e.g., the individual units run sequentially withoutput (effluent stream) from one unit being passed on to another unitfor further treatment, e.g., filtrate from the UF unit being furthertreated in the DCMD, dilute draw from a FO unit being further treated ina DMCD unit to produce desalinated water and regenerate the draw forre-use in the FO unit, etc. In another embodiment the system isconfigured to run in parallel mode, e.g., filtrate from the UF unit issplit for processing in both the DCMD and FO units. In yet anotherembodiment, the system is configured for some of the units to be online(e.g., the DCMD unit running) and some units being off-line not in use(e.g., FO unit not being used). The system can also be configured to berunning in both parallel and sequential modes, e.g., multiple UF unitsbeing employed in series and both DCMD and FO units being used for theremoval of dissolved organics, etc.

Reference will be made to the figures that schematically illustratevarious embodiments of different configurations for the treatment ofproduced water, particularly for steam generation.

FIG. 1 illustrates an embodiment of a process and a system that employselectrocoagulation to treat water to produce steam at significant energysavings. In the system, produced water feed 11 passes through screen 10for the removal of large particles that may damage downstream equipment.The screened feed stream 21 is treated in EC unit 20, for the removal ofat least 95% of the hardness and 90% of the silica respectively. In oneembodiment, additional additives 22 such as flocculants, oxidizingagents and the like can also be added to the EC unit 20. In anotherembodiment, the pH of the screened feed stream 21 is adjusted to apre-select pH with the addition of at least a base 12 prior to treatmentin the EC unit. A clarifier 30 (or other applicable solid removal units)is used for the removal of any flocs or precipitates as wastestream 32.The clarified water 41 is sent to a high-temperature filtration unit 42for the removal of any free oil 42, generating a treated water streammeeting specs for hardness and silica for use in steam generation.

FIG. 2 is an alternative configuration, wherein instead of an EC unit, achemical precipitation (CP) unit 25 is employed with the addition of anyof lime 12, caustic, and/or magnesium compounds 26, and wherein aceramic UF unit 45 is used for high-temperature filtration to remove atleast 90% of the free oil in the treated water stream 41.

FIG. 3 is yet another variation of the configuration in FIG. 1, whereina high temperature polymer UF unit 47 is employed to remove free oil inthe treated water stream 41 prior to steam generation.

FIG. 4 is a variation of the configuration in FIG. 3, wherein thetreated stream 43 is further processed in a DCMD unit 50 for the removalof dissolved organic constituents, generating a high quality treatedstream 51 for steam generation.

FIG. 5 is yet another variation of the configuration in FIG. 3, whereinthe treated stream 43 is sent to a FO unit 55 (instead of a DCMD unit)for the removal of dissolved organic constituents, generating a highquality treated stream 51 for steam generation.

FIG. 6 employs the same system configuration in FIG. 5 for the treatmentof produced water. However in this embodiment, an untreated producedwater stream 52 having a low concentration of TDS can be combined withstream 43 and fed directly to the FO unit for further treatment. In oneembodiment, the ratio of untreated water stream 52 to treated waterstream 43 is controlled such that the final treated water stream 51remains within spec.

In the configuration of FIG. 7, the produced water has a sufficientlylow silica level to start and only hardness removal is needed. Afterpretreatment with screening in unit 10 and ceramic UF unit 45, thetreated water 43 is desalinated in DCMD unit 50, generating a treatedstream 43. An untreated produced water stream 52 having a lowconcentration of TDS can be combined with treated stream 431 for furthertreatment in IE unit 60 to remove hardness ions to a concentration ofless than 1 ppm.

As shown in some figures, pretreatment in the EC unit 20 and/or UF unitis desirable before employing a MD step to prevent flux decline andscale build-up. The EC unit can improve the efficiency of the membranedistillation flux by reducing the scalants (silica and hardness) andfoulants (free oil and dissolved organics). In embodiments with minimumgain/loss of energy in steam generation, the use of EC is particularlysuitable. When operated at reasonably high temperatures, the EC processdesirably adds energy to the produced water. The EC process raises thetemperature of the water, which increases the driving force in thesubsequent membrane distillation step. Thus, there is a synergisticcombination in providing a pretreatment step of EC, followed by an MDprocess. The energy the EC process adds to the water assists in“driving” the membrane distillation process.

The use of high-temperature ultra-filtration is also desirable forembodiments with minimum gain/loss of energy in steam generation. In oneembodiment with the use of ceramic or high-temperature polymer materialsfor membranes, the clarification step can be eliminated with the solidremovals being carried out in the ultra-filtration step. In otherembodiments, ultra-filtration may be bypassed if the membranedistillation unit is tolerant to residual suspended solids and residualdissolved organics after the EC treatment.

It should be noted that separate water treatment units are configured ina permutable fashion for the system to be operating according to any ofthe configurations described above depending on the properties of theproduced water source, with some of treatment units to be online, someon stand-by mode, parallel mode with water treatment by both EC and FOunits, series mode with ceramic UF filtration prior to treatment by theDCMD, split feed treatment with some of the produced feed by-passing oneor more of the treatment units, etc.

EXAMPLES

The invention is shown by example in the illustrated embodiments.However, it is recognized that other embodiments of the invention havinga different configuration but achieving the same or similar result arewithin the scope and spirit of the claimed invention.

Example 1 Pretreatment by Electrocoagulation (EC)—Produced Water

Produced water from a oil producing field was collected and analyzed forits water quality shown below:

mg/L Anions Bicarbonate, HCO₃ ⁻¹ 1395 Carbonate, CO₃ ⁻² 0.0 Chloride,Cl⁻¹ 3940 Hydroxide, OH⁻¹ 0.0 Sulfate, SO₄ ⁻² 177 Sulfide, S⁻² 0.0Sulfite, SO₃ ⁻² 0.0 Cations Ammonium, NH4⁺¹ 0.00 Barium, Ba⁺² 0.76Boron, B⁺³ 92.8 Calcium, Ca⁺² 50.9 Iron, Fe⁺³ 0.00 Magnesium, Mg⁺² 16.4Potassium, K⁺¹ 115 Sodium, Na⁺¹ 2540 Strontium, Sr⁺² 0.00 Silica, asSiO2 293.0 Sodium, Na⁺¹ (Calc.) 2541.0 Chloride, Cl⁻¹ (Calc.) 4132.0

This water is high in silica and hardness. It was treated in a ECreactor at an applied current of 10-15 A and a pH range of 7.5-9.5. Theresults are shown in Table 1, showing a substantial reduction in silica,calcium, and magnesium.

TABLE 1 Untreated Produced Treated Produced % Contaminant Water (ppm)Water (ppm) Reduction Silica (SiO2) 293 15 95 Calcium 50.9 4.6 91Magnesium 25.9 2.1 87

Example 2 EC Pretreatment—Tar Sand Wastewater

EC process has been shown in sources to treat tar sand wastewater as thefeed. Results are shown in Table 2, showing a large decrease in totalsuspended solids and total organic carbon.

TABLE 2 Contaminant % Removal Total suspended solids 99 Total organiccarbon 50-95

Example 3 EC Pretreatment—Oil Shale Wastewater

Example 1 was repeated but with oil shale wastewater as the feed. Thecontaminant removal percentage of dissolved organic carbon was about17-36 percent removal.

Example 4 Effect of Temperature

The produced water sample from Example 1 was run through the EC benchscale unit at a feed water temperature of 16° C. The temperature of thetreated water increased due to energy input from the EC process as shownin Table 3.

TABLE 3 Power Input Impact on Treated Water Temperature Current PowerTreated Water Increase in Run (A) (W) Temperature ° C. temperature, ° C.1 11 660 28 12 2 9 495 27 11 3 7 343 23 7 4 5 125 22 6 5 3 60 21 5 6 111 19 3

Example 5 Flux Performance in DCMD

From various literature sources, as shown in Table 4, high water fluxcan be achieved. This table shows water flux as a function of the TDSand driving force. It is shown to be high for membrane distillation, anddesirably can be applied with very high TDS water where reverse osmosismay not be applicable. It also shows that the flux is independent of thefeed water TDS. The results further indicate that the process can beapplied for many applications in the shale gas reservoir productionindustry such as in desalinating flowback water which have a saltconcentration of at least 6 wt % or more.

TABLE 4 Water Source Driving Force (Delta C) Flux (gfd) City Water (lowTDS) 70 28.6 Brine (6 wt %) 65 11.5 Brine (10 wt %) 65 11.5 Brine (6 wt%) 45 6.2

Example 6 FO Performance

In another example, produced water from Example 1 was treated by FOmembrane for 24 hours. 1 L of 1.25 M NaCl was used as draw solution torecover over 45% of produced water at an average flux of 8.1 LMH. Fortwo consecutive experiments, no flux drop was observed, indicating lowor no fouling. The results in Table 5 show significant reduction intotal hardness by FO treatment. With respect to removal of dissolvedorganic carbon, the feed water shows an initial concentration of 441mg/L, the concentrated feed water has a concentration of 780 mg/L, andthe feed water shows a final concentration of 2.35 mg/L. Thus forwardosmosis membranes can be seen to reduce hardness and scale causingconstituents of produced water. In some applications, extensivepretreatment may not be required. FO osmosis can useful in applicationsfor desalinated water wherein boron reduction is desired.

TABLE 5 Concentration Feed water (initial mg/L) Draw water (final mg/L)Ca 50.9 1.1 Mg 25.9 2.1 Silica 293 9.4 Boron 92.8 8.8

Example 7

In an example of a produced water flow rate of 150,000 BWPD (barrels ofwater per day) for steam generation purpose, a polymeric UF membrane isused for the removal of dissolved organics. The produced watertemperature is reduced from ˜75° C. to 45° C. before being introducedinto the UF unit, wherein the heat content is transferred to anothermedium via heat exchanger and transferred back to the water after thedissolved organics are separated/removed. For a typical heat exchangerefficiency of 85%, it is estimated that about 320,000 MM BTU is lost, oran operational loss of about $1.6 million a year at a natural gas priceof $5/MM BTU.

Example 8

Example 7 is repeated but with the use of ceramic membranes for theremoval of organics from a produced water flow rate of 150,000 BWPD(barrels of water per day) for steam generation purpose. Produced waterat ˜75° C. can be fed directly into the UF unit for oil removal forsubsequent steam generation purposes, for a saving of at least $1.6million a year as there is no need for heat exchanger systems.

For the purposes of this specification and appended claims, unlessotherwise indicated, all numbers expressing quantities, percentages orproportions, and other numerical values used in the specification andclaims, are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that can vary depending upon thedesired properties sought to be obtained by the present invention. It isnoted that, as used in this specification and the appended claims, thesingular forms “a,” “an,” and “the,” include plural references unlessexpressly and unequivocally limited to one referent. As used herein, theterm “include” and its grammatical variants are intended to benon-limiting, such that recitation of items in a list is not to theexclusion of other like items that can be substituted or added to thelisted items.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope is defined bythe claims, and can include other examples that occur to those skilledin the art. Such other examples are intended to be within the scope ofthe claims if they have structural elements that do not differ from theliteral language of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims. All citations referred herein are expressly incorporatedherein by reference.

The invention claimed is:
 1. A method for treating produced water for steam generation, the method comprising: providing a source of produced water to be treated, the water having contaminants selected from the group of total organic content (“TOC”) and total dissolved solids (“TDS”) as silica and hardness ions; removing at least a portion of the silica and hardness ions as suspended solids by subjecting the produced water to an electrocoagulation process; removing at least a substantial portion of the suspended solids in a polymer filtration unit, the filtration unit operating at a temperature of at least about 50 degrees Centigrade; lowering the water temperature of the produced water; further treating the produced water within the filtration unit, further wherein the filtration unit comprises at least one membrane selected from the following: polyethersulfone (PES) membrane, polyacrylonitrile (PAN) membrane, polyvinylidene difluoride (PVDF) membrane, sulfonated polyether ether ketone (PEEK) membrane, Nafion membranes (sulfonated tetrafluoroethylene based fluoropolymer-copolymer), poly(phthalazinone ether sulfone) membrane (PPES), poly(phthalazinone ether ketone) membrane (PPEK), poly(phthalazinone ether fulfone ketone) membrane (PPESK); generating a pre-treated water; passing the pre-treated water to a forward osmosis (FO) unit employing a draw solution selected comprising at least one of polyvalent osmotic ions, monovalent osmotic ions and combinations thereof; subsequently treating the pre-treated water in a direct contact membrane distillation (DCMD) unit, wherein the DCMD unit employs a composite membrane comprising a hydrophilic polymer layer and a hydrophobic polymer layer.
 2. The method of claim 1, wherein no heat energy is added to or removed from the pre-treated water prior to passing the pre-treated water to a forward osmosis unit.
 3. The method of claim 1, wherein the draw solution comprises at least one of NaCl, Na₂SO₄, AlCl₃, MgSO₄, NH₄HCO₃, MgCl₂ and mixtures thereof.
 4. The method of claim 1, wherein the electrocoagulation process employs sacrificial electrodes.
 5. The method of claim 1, wherein the pH of the produced water is adjusted to a pre-select pH prior to removing at least a portion of the silica and hardness ions by the electrocoagulation process.
 6. The method of claim 5, wherein the pre-select pH ranges from 7.2 to 11.5.
 7. The method of claim 1, wherein the filtration unit to removes at least 70% of the TOC as free oil prior to passing the pre-treated water to a forward osmosis unit.
 8. The method of claim 1, wherein the filtration unit employs additionally comprises a ceramic membrane to remove at least 70% of the TOC as free oil.
 9. The method of claim 1, further comprising passing the produced water through a screen to remove large particulates prior to subjecting the produced water to an electrocoagulation process.
 10. The method of claim 1, further comprising: passing a dilute draw from the FO unit to the direct contact membrane distillation (DCMD) unit to generate a treated stream and a draw stream for re-use in the FO unit.
 11. The method of claim 1, wherein no heat energy is added to or removed from the at least a portion of the pre-treated water prior to treating in the DCMD unit.
 12. The method of claim 1, wherein the DCMD unit employs at least a hydrophobic hollow fiber membrane.
 13. The method of claim 12, wherein the membrane fibers have a length ranging from 1 to 200″, a wall thickness ranging from 2 to 100 μm.
 14. The method of claim 1, wherein the hydrophilic polymer comprises any of polysulfone, polyether sulfone, polyetherimide polyvinylidenefluoride, and cellulose acetate.
 15. The method of claim 14, wherein the hydrophobic polymer layer comprises fluorinated surface-modifying macromolecule (SMM). 